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Generic lists
f#

I just started learning F# and I'm a little puzzled by an error I'm receiving:

"The type 'Microsoft.FSharp.Collections.list`1' expects 1 type of argument(s) but is given 0"

I created the following code:

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let concatList1 x y = x @ y
printfn "%A" (concatList1 ["abc"; "def"] ["ghi"; "jkl"])
printfn "%A" (concatList1 [1; 2] [3; 4])

The code just concatenates two lists together and works as expected. Then I changed the code to include type information:

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let concatList2 (x:list) (y:list) = x @ y
printfn "%A" (concatList2 ["abc"; "def"] ["ghi"; "jkl"])
printfn "%A" (concatList2 [1; 2] [3; 4])

This is where I get the message. It appears that you cannot say you have a generic type of list. You need to specify a "type" of list, e.g., (x: string list). I looked through the language reference multiple times, but didn't see how you could type x and y to be a generic list, like was done with the type inference in concatList1. Am I missing something or is this a limitation in the language.

Thanks, Andy.

.

Any of these will work:

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let concatList2 (x:_ list) (y:_ list) = x @ y
let concatList2 (x:list<_>) (y:list<_>) = x @ y
let concatList2 (x:'a list) (y:'a list) = x @ y
let concatList2 (x:list<'a>) (y:list<'a>) = x @ y

Since list is a generic type, it needs to take a type argument. You can either use the prefix ('a list) or postfix (list<'a>) forms. "_" asks the compiler to infer it for you. Any identifier starting with an apostrophe is a generic type parameter (like <T> in C#).

By on 9/30/2008 4:21 PM ()

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